172 GENERAL DEVELOPMENT. 



General development of the Embryo. 

 The formation of the embryo commences with the appearance of 

 the medullary plate, the sides of which 

 soon grow up to form the medullary folds. 

 ,,,;f JV^^ '^^^^ jnedullary groove is developed an- 



teriorly before any trace of it is visible 

 behind. In a general way the closure of 

 the groove takes place as in Birds, but 

 ;> j the anterior part of the body is very early 

 I 111 folded off, sinks into the yolk, and becomes 

 I jlf covered over by the amnion as by a hood 

 i' (figs. 127 and 129). All this takes place 



-2^y before the closure of the medullary canal; 

 , ,- and the changes of this part are quite 



'|:^ '0 concealed from view. 



NiiiiiiHB ' The closure of the medullary canal 



Fig. 129. Surface view of commences in the neck, and extends for- 

 A YOUNG EMBRYO OF Lacerta ^^^^^ ^^^ backwards) aud thc whole 



MUKALIS. . p ,1 1 • 1 1 J • 



am. amnion; pr. primitive ^^g\0>^ "^ ^^e brain becomes closed ID, 



streak. wbile the groove is stiM largely open 



behind. 

 The later stages in the development of the Lacertilian embryo 

 do not require a detailed description, as they present the closest 

 analogy with those already described for Aves. The embryo soon 

 turns on to its left side; and then, becoming continuously folded off 

 from the yolk, passes through the series of changes of form with 

 which the reader is already familiar. An advanced embryo is repre- 

 sented in fig. 130. The early development and great length of the 

 tail, which is spirally coiled on the ventral surface, is a special feature 

 to which the attention of the reader may be called. 



Embryonic Membranes and Yolk-Sack. 



The early development of the cephalic portion of the amnion has 

 already been alluded to. The first traces of it become apparent 

 while the medullary groove is still extremely shallow. The medullary 

 plate in the region of the head forms an axial strip of a thickish plate 

 of epi blast. The edge of this plate coincides with the line of the am- 

 niotic fold, and as this fold rises up the two sides of the plate become 

 bent over the embryo and give rise to the inner limb of the amnion 

 or amnion proper. The section (fig. 127), representing the origin of 

 the amniotic hood of the head, shews very well how the space between 

 the two limbs of the amnion is continuous Avith the body cavity. 

 The amnion very early completely encloses the embryo (fig. 128 A 

 and B), and its external limb or serous membrane, after separating 

 from the true amnion, soon approaches and fuses with the vitelline 

 membrane. 



The first development of the allantois as a diverticulum of the 

 hypoblast covered by splanchnic mesoblast, at the apparent posterior 



